1. Field of the Invention
This invention relates to an improved offshore structure for use in an arctic environment. More particularly, this invention relates to an offshore structure having a multiple slope surface which controls the impinging horizontal forces due to moving ice sheets of various thicknesses.
2. Description of the Prior Art
The offshore polar regions of the world, particularly the arctic, are hostile environments for offshore structures. Continuously moving ice masses pose a threat to the successful operation of an offshore structure. Ice sheets and ice floes of 10 feet or more in thickness are fairly common in these polar regions. Occasionally, these sheets or floes crush against one another and produce an irregular ridge of ice rubble commonly referred to as a pressure ridge. If a pressure ridge is sufficiently large it may survive the summer melt period and become a multiyear pressure ridge. Investigations have revealed multiyear pressure ridges consisting of solid ice more than 100 feet thick.
These ice floes and ridges cover most of the Arctic Ocean. They move relatively slowly; however, when they move against a stationary offshore structure they may exert very large forces on it. The load from large formations of solid ice such as multiyear pressure ridges will generally determine the design load for the structure. Conical structures are designed so as to break ice by bending it upward and deflecting it around the structure.
In simple terms, there are three basic types of ice failures possible: crushing, shearing and bending. Crushing failures exert the greatest force on a structure due to the high compressive strength of ice (500 to 1000 pounds per square inch (psi)). For this reason, it is desirable to avoid crushing failures of the ice. This can be accomplished by providing a structure which has a slanting or sloping exposed surface at and below the waterline. This slanting or exposed surface bends the ice as it impinges on the structure and causes a bending failure. The shearing failure generally occurs after the crushing mode failure has begun.
Previous conical structures proposed for the arctic offshore typically employ a single-slope surface to fail the ice in bending. Sloped surfaces produce a bending mode failure because the ramp-like surface causes the edge of the moving ice to be forced upward, and it has generally been found that the horizontal forces exerted on a sloped structure are substantially less than the forces exerted against a vertically oriented surface.
Conical structures having a single slope below the waterline have been used as lighthouses and light piers in Lake Erie and along the St. Lawrence River for many years (see "Effect of Cone-Shaped Structures on Impact Forces of Ice Floes", by Danys, J. V., Proceedings of Port and Ocean Engineering Under Arctic Conditions, Trondheim, Norway, 1971, Vol. 1, pp 609-620). Additional single-slope configurations for resisting ice forces are illustrated in U.S. Pat. Nos. 3,645,104; 3,745,777; 3,793,840; 3,831,385 and 3,952,527. These references disclose a single slope which extends from substantially the sea bottom to a deflector zone at or near the water surface. When installed for use in the arctic, these structures would, typically, have to be designed for the load that a relatively thick pressure ridge would exert; however, they would then be overdesigned near the water surface for the more common relatively thin ice sheets which exert lesser loads. Therefore, the need exists for an improved offshore structure which is designed to handle the loads of thick as well as thin ridges without being overdesigned.